This invention relates to video signal processing and in an important example to improved methods of processing television signals which have been derived from cinema film, or other camera processes having a temporal sampling rate lower than the field rate of the television system. The invention also relates more generally to the detection of motion in a video signal.
There are several processes in which two or more fields of a television signal are arithmetically combined to provide a filtered or interpolated output signal. Examples include geometric transformation of the picture for special effects, aspect-ratio conversion, composite decoding and standards conversion. These processes can give rise to undesirable artefacts when the fields that are combined differ significantly because of motion, or cuts between different scenes. When the temporal sampling rate of the camera is less than the field rate of the television system the opportunity arises to modify the processing so that only fields corresponding to the same instant in time are combined.
A particularly important example of the problem is the televising of film shot at 24 frames per second at a field rate of 60 fields per second. It is common practice to create a sequence of five television fields from two film frames by alternately generating two and three fields respectively from successive film frames. This is known as the “3:2 pull-down” technique. Techniques have been developed for identifying the duplicated fields by comparing fields one frame apart and analysing the resulting pattern to derive a film sequence signal. Reference is directed in this respect to U.S. Pat. Nos. 4,881,125; 4,982,280; 4,998,287 and 5,255,091.
A difficulty with prior art techniques is that they rely on analysing data from many frames of video to identify characteristic patterns. Unless there is considerable delay built into the system, it is difficult to react quickly to changes in temporal phase.
When film shot at 24 frames per second is televised at a field rate of 50 fields per second, the so-called 2:2 technique is employed. Here, every film frame is used to provide two video fields and the tape or film transport speed is modified to change the resulting 48 fields per second signal into the required 50 fields per second. There remains a need to identify in the video signal, which fields originate from the same film frame.
Many processes which combine fields are arranged to operate either in a “video mode” where each field is assumed to correspond to a different moment in time, or a “film-mode” where successive fields may correspond to the same moment in time. It is therefore useful to create a film/video control signal and, in order to modify processes which combine fields inappropriately, it is helpful to derive a signal which indicates when a change in the “temporal phase” of the picture, or a cut to a new scene, occurs.
A proposal has been made to detect motion between interlaced video fields to provide a field motion signal: see U.S. Pat. No. 5,291,280. In this proposal, a signal formed by subtracting across a field delay is compared with a signal formed by subtracting across a delay of a field less one line. After filtering, rectifying and thresholding, the smaller of these two signals is taken as the field motion signal.
Interpreting a difference taken across a field delay is complicated by the common practice of interlaced television scanning. The lines of successive fields are vertically misaligned by one line pitch and so, where vertical detail exists, the magnitude of the difference signal will not fall to zero, even if the fields correspond to the same scene and temporal phase.
It is an object of this invention to provide improved method and apparatus that overcome certain shortcomings of the prior art.